EP4266082A1 - Control circuit for a piezoelectric ultrasonic transducer and ultrasonic transducer system - Google Patents

Abstract

The invention relates to a control circuit (2) for a piezoelectric ultrasonic transducer (4) in an ultrasonic transducer system (1), comprising:
- a transformer (21) with at least one primary winding (22, 22', 22");
- a switching unit with a semiconductor switch (24', 24"), which is connected to the at least one primary-side winding (22, 22', 22") via a switched connection (A', A"),
- a control unit (5) which is designed to alternately apply an operating voltage (U _B ) to or disconnect it from the at least one primary-side winding (22, 22', 22"),
- a protective circuit (25) which is electrically coupled to the switched connection (A', A") and has a Zener diode (ZD1) which sets a switch-off voltage at the switched connection (A', A") to a limiting voltage (U _G ) which corresponds to at least twice the operating voltage.

Description

Technical area

The invention relates to control circuits for piezoelectric ultrasonic transducers with improved protection against overvoltages due to switching processes on transformer inductances.

Technical background

Ultrasonic transducers are used for object detection and usually have a piezo actuator that requires a high electrical alternating voltage of around 100 to 200 V for excitation. The voltage level of such a control voltage is usually provided by voltage transformation using a transformer in the final stage.

During transmission mode, a periodic pulsed operating voltage is applied to the transformer on the primary side, which is accordingly transformed up to the desired control voltage. The excitation takes place in pulse form for reasons of simpler circuit design. For pulse-shaped excitation of such a transformer, an operating voltage provided is switched on or off on the primary side. The pulse-shaped control of the transformer results in a relatively high switch-off voltage due to the electrical energy stored in the inductance of the primary-side transformer winding. This effect is also known as the flyback effect.

The high switch-off voltage on the primary side of the transformer resulting from the flyback effect must be limited to prevent damage to components Avoid control circuit. As a rule, the energy of the primary-side inductance is derived via diodes and dissipated as power loss in the form of heat.

Ultrasonic transducer systems usually control the ultrasonic transducer alternately in a transmission mode for emitting an ultrasonic transmission signal and a reception mode for receiving an ultrasonic reception signal that corresponds to an ultrasonic transmission signal reflected on one or more surrounding objects. The resulting ultrasonic signal transit times, along with other features, are evaluated in order to obtain information about environmental objects located in the detection range of the ultrasonic transducer system. When switching from transmitting mode to receiving mode, it is necessary to completely dissipate the energy stored on the primary side as quickly as possible in order to provide the sensitivity at the beginning of the receiving mode as quickly as possible after the end of the transmitting mode. Establishing the detection capability after the end of the transmission phase largely determines the distance up to which surrounding objects can be detected by the ultrasonic transducer system.

It is the object of the present invention to provide a control circuit for an ultrasonic transducer and an ultrasonic transducer system that is more efficient and enables improved performance for detecting surrounding objects in the close range.

Disclosure of the invention

This task is solved by the control circuit for an ultrasonic transducer according to claim 1 and by the ultrasonic transducer system according to the independent claim.

Further refinements are specified in the dependent claims.

• einen Transformator mit mindestens einer primärseitigen Wicklung;
• eine Schalteinheit mit einem Halbleiterschalter, die mit der mindestens einen primärseitigen Wicklung über einen geschalteten Anschluss verbunden ist,
• eine Steuereinheit, die ausgebildet ist, um eine Betriebsspannung alternierend an die mindestens eine primärseitige Wicklung anzulegen oder davon zu trennen,
• eine Schutzschaltung, die mit dem geschalteten Anschluss elektrisch gekoppelt ist und eine Zenerdiode aufweist, die eine Abschaltspannung am geschalteten Anschluss betragsmäßig auf eine Begrenzungsspannung begrenzt, die mindestens der doppelten Betriebsspannung entspricht.

According to a first aspect, a drive circuit for a piezoelectric ultrasonic transducer is provided in an ultrasonic transducer system, comprising:

• a transformer with at least one primary winding;
• a switching unit with a semiconductor switch, which is connected to the at least one primary-side winding via a switched connection,
• a control unit which is designed to alternately apply an operating voltage to or disconnect from the at least one primary-side winding,
• a protective circuit that is electrically coupled to the switched connection and has a Zener diode that limits the amount of a switch-off voltage at the switched connection to a limiting voltage that corresponds to at least twice the operating voltage.

It can be provided that the limiting voltage is selected from a range between 100 and 150% of the amount of twice the operating voltage, in particular between 105 and 130% of the amount of twice the operating voltage.

Furthermore, the limiting voltage can be determined by selecting a threshold voltage or breakdown voltage of the Zener diode.

Preferably, the protective circuit can be electrically non-capacitively coupled to the at least one switched connection.

In a control circuit for an ultrasonic transducer, a transformer is provided for transmission operation in order to be able to control the ultrasonic transducer, which is designed in particular as a piezo actuator, with correspondingly high control voltages. The control takes place with an alternating voltage, which is usually provided by a pulsed operating voltage provided on the primary side of the transformer. The pulsed operating voltage is created by applying or switching off an operating voltage, in particular using suitable semiconductor switches, such as. B. field effect transistors or the like.

Due to the inductance of the at least one primary-side winding of the transformer, when the operating voltage is switched off, a significant voltage increase occurs at the switched transformer connection due to the electrical energy stored in the inductance. The increase in voltage can lead to damage or destruction of components of the ultrasonic transducer system. Therefore A suitable protective circuit is usually provided which limits the voltage increase after the primary winding is switched off.

Immediately after the primary-side winding of the transformer is switched off, twice the operating voltage is usually present at the switched transformer connection, since the potential of the corresponding other transformer connection of the primary-side winding is not switched and is therefore kept constant. Without further measures, the switch-off voltage across the primary-side winding would continue to rise due to the electrical energy stored in the winding if a current flow from the primary-side winding is severely limited or even prevented.

According to the above control circuit, the protective circuit is therefore coupled to the switched connection of the at least one primary-side winding and has a Zener diode. The starting voltage (breakdown voltage) of the Zener diode is intended to be at least twice the operating voltage with which the primary-side winding is switched. By setting the threshold voltage of the Zener diode to a value above twice the operating voltage, an increase in the switch-off voltage across the primary-side winding beyond a voltage threshold defined by the threshold voltage is efficiently prevented. In this way, it is possible, firstly, to limit the voltage across the primary winding to protect the other components and, secondly, to select this voltage in such a way that it ensures optimal energy dissipation in interaction with the surrounding components.

Due to the targeted flow of current in the switch-off phase via the protective circuit, less electrical energy remains in the transformer during the transition from transmitting mode to receiving mode, so that the decay process at the end of transmitting mode is significantly shortened. Any ultrasonic received signal received by the ultrasonic transducer can thus be detected in an improved manner, since the ultrasonic received signal is not superimposed by any decay signal after the end of the transmission operation or is only superimposed for a shorter period of time.

It can be provided that the operating voltage is buffered by a buffer capacity, with the protective circuit being coupled to the buffer capacity via a leakage resistor. In this way, a charge reservoir can be created for one at the beginning of the The high inrush current required for the primary winding to be switched on can be provided. This return of the energy dissipated when switching off into the supply circuit makes it possible to reduce the power requirement of the control circuit.

Furthermore, the protective circuit can be coupled to the at least one switched connection via a diode, in particular directly via a diode, which, when a voltage difference occurs between the at least one switched connection and the Zener diode, limits a charge flow from or to the at least one switched connection a voltage change at the at least one switched connection.

The protective circuit can thus be decoupled by a serial connection with a diode, so that the capacitance input of the protective circuit on the switched transformer connection or on the primary winding of the transformer is reduced. In addition, the series-connected diode allows the Zener diode to be biased so that its switching inertia is minimized.

It can also be provided that a storage capacity is electrically connected in parallel to the Zener diode, which supplements the capacity of the Zener diode. This enables improved temporary storage of the electrical energy dissipated after the primary winding has been switched off, which is needed for the next switching cycle, i.e. H. can be used the next time the primary winding is switched on. Because the starting voltage of the Zener diode is specifically chosen to be higher than twice the operating voltage, when the switch-off voltage increases, the resulting current flow can flow through the primary winding into the storage capacity. This continues until the breakdown voltage of the Zener diode is reached. The current flow then continues into these until their breakdown voltage falls below again. At the same time, the extended flow also results in an increase in the secondary side voltage.

The storage capacity is particularly useful in conjunction with the diode provided in series with the protective circuit, which decouples the protective circuit from the transformer winding. The diodes are therefore used to To decouple the capacity formed by the Zener diode and the storage capacity from the transformer circuit and to apply a bias voltage that increases the switching speed.

In addition, the capacity to absorb the outflowing charge is increased, so that the capacitance of the Zener diode no longer has a significant influence on the switching speed, but at the same time enables the energy flowing back from the transformer to be stored. The energy stored in this capacity can then be fed back into the charge reservoir of the buffer capacity via the leakage resistor.

According to one embodiment, the transformer can have two primary-side windings, which are alternately connected to and disconnected from the operating voltage, so that the operating voltage is always only present on one of the primary-side windings.

The corresponding switched connections of the primary-side windings can in particular be connected to the protective circuit via a respective diode.

• einen piezoelektrischen Ultraschallwandler;
• die obige Ansteuerschaltung, wobei eine sekundärseitige Wicklung des Transformators mit dem Ultraschallwandler verbunden ist.

According to a further aspect, an ultrasonic transducer system is provided comprising:

• a piezoelectric ultrasonic transducer;
• the above control circuit, wherein a secondary winding of the transformer is connected to the ultrasonic transducer.

Brief description of the drawings

Fig. 1

eine schematische Darstellung eines Schaltbilds eines Ultraschallwandlersystems mit einer Ansteuerschaltung; und

Fig. 2

ein Signal-Zeit-Diagramm von Spannungssignalen der Ansteuerschaltung.

Embodiments are explained in more detail below with reference to the accompanying drawings. Show it:

Fig. 1

a schematic representation of a circuit diagram of an ultrasonic transducer system with a control circuit; and

Fig. 2

a signal-time diagram of voltage signals from the control circuit.

Description of embodiments

Figure 1 shows an ultrasonic transducer system 1 with a control circuit 2 for a transmission operation of the ultrasonic transducer system 1 and a reception circuit 3 for a reception operation of the ultrasonic transducer system 1. The control circuit 2 and the reception circuit 3 are connected to an ultrasonic transducer 4. The receiving circuit 3 is only shown schematically, and the specific implementation will not be discussed further here.

The ultrasonic transducer 4 has a piezo actuator that can be controlled with high piezo voltages between 100 and 200 V in an ultrasonic frequency range in order to emit an ultrasonic transmission signal during transmission operation. An ultrasound reception signal, which is created by a reflection of the ultrasound transmission signal on one or more surrounding objects, leads to a change in capacitance that can be detected in the reception circuit 3 in a reception mode.

The control circuit 2 and the reception circuit 3 are generally operated in an alternating transmission and reception mode via a control unit 5.

The control circuit 2 includes a transformer 21, which is coupled to the ultrasonic transducer 4 on the secondary side.

The transformer 21 has two primary-side windings 22 and one secondary-side winding 23. The primary-side windings 22 are alternately energized with a pulsed operating voltage that corresponds to the desired control frequency of the emitted ultrasonic transmission signal. The primary-side windings 22 are connected to one another with a common center connection M, which is connected to a first supply potential U _B (operating voltage). In particular, the coupling to the first supply potential U _B can take place via a resistor R2 and a capacitor C4, which provides a capacitance with respect to a second supply potential GND, in particular a ground potential.

A first switched connection A' of the first of the primary-side windings 22' is connected via a first semiconductor switch 24' to the second supply potential GND and a second switched connection A" of a second of the primary-side windings 22". a second semiconductor switch 24" is connected to the second supply potential GND. The semiconductor switches 24', 24" are alternately controlled using the control unit 5 according to the control frequency for opening or closing with a respective control signal S', S", so that the first and the second primary-side winding 22 ', 22" is energized and the other is separated from the operating voltage between the first supply potential U _B and the second supply potential GND.

The secondary-side winding 23 is connected to the ultrasonic transducer 4, which can optionally be buffered with a buffer capacitor CR1. Due to the high switching currents that occur when the operating voltage is applied to the primary-side windings 22', 22", the first supply voltage can be buffered with a buffer capacity C2, which acts as a charge reservoir for high initial currents after one of the primary-side windings 22', 22" is switched on. serves.

The switched connections A', A" of the first and second primary-side windings 22', 22" are connected to a protective circuit 25 via a respective diode D1 and D2. The forward directions of the diodes D1 and D2 each correspond to a positive voltage between the switched connections A', A" and the protective circuit 25.

The protective circuit 25 has a Zener diode ZD1, which is connected via the diodes D1 and D2 to the switched connection A ', A "and to the second supply potential GND. The threshold voltage (breakdown voltage) U _ZD of the Zener diode ZD1 corresponds to a voltage that is at least is set to twice the operating voltage, in particular slightly more than twice the operating voltage, in particular between 100% and 150%, preferably between 105% and 130%, of twice the operating voltage. The starting voltage of the Zener diode thus defines a limiting voltage for the voltage at the switched connection A', A".

Due to the alternating operation of the primary-side windings 22', 22", a switch-off voltage or a switch-off potential that is approximately twice the potential of the first supply potential U _B (based on the second Supply potential GND). The switch-off potential is applied to the protective circuit 25 via the diodes D1, D2 (minus the diode voltage). The limitation of the voltage increase for the primary-side windings 22', 22" of the transformer 21 or the electronic semiconductor switch (transistor, FET, ...) 24', 24" connected to it is formed from the starting voltage U _ZD of the Zener diode plus the diodes -Forward voltage of diodes D1 and D2. If the Zener diode ZD1 is used without the diodes D1 and D2, the limitation of the voltage rise corresponds to the starting voltage of the Zener diode ZD.

The protective circuit 25 limits the corresponding switch-off potential to the potential of the threshold voltage of the Zener diode ZD1. Without the Zener diode ZD1, the switch-off potential would continue to increase, since the stored electrical energy causes a current flow through the primary-side winding 22', 22", which acts back on the first supply potential VB or leads to a sharp increase in the voltage at the corresponding switched connection A' "A" would lead.

The Zener diode ZD1 limits the voltage increase to the threshold voltage.

A storage capacity is preferably connected in parallel to the Zener diode ZD1. The parallel connection increases the charge-storing capacity, which takes over part of the amount of charge flowing out from the switched-off primary-side winding 22', 22" and is charged to a voltage level that corresponds to the starting voltage of the Zener diode ZD1.

Via a leakage resistor R1, with which the protective circuit 25 is connected to the first supply potential VB, the charge from the storage capacity C1 can now be supplied to the first supply potential VB, in particular there into the buffer capacity C2, in order to correspondingly supply the charge for the storage capacity C1 in a subsequent switching process to provide high initial current. As a result, the current requirement of the control circuit 2 can be reduced, since part of the electrical energy derived when the corresponding primary winding is switched off can be used by storage in the storage capacity C1 and the buffer capacity C2 for a corresponding subsequent switching process with the required high initial current.

The diodes D1, D2 have the advantage that the capacity of the Zener diode ZD1 and the optional storage capacity C2 are decoupled from the switched connections A', A", so that their capacitance input into the switching of the transformer 21 is reduced. On the other hand, the Zener diode the protective circuit 25 is biased via the leakage resistor R1, so that its switching inertia is reduced. The diodes decouple this part of the protective circuit from the primary-side windings and enable this biasing.

The protective circuit 25 also makes it possible to quickly dissipate the electrical energy from the primary-side windings 22 ', 22 ", so that less electrical energy remains in the transformer 21, and in particular after the end of the transmission operation, the transformer-ultrasonic transducer combination, which includes a resonant circuit, swings out the inductance of the secondary-side winding 23 and the capacitance of the piezo actuator of the ultrasonic transducer 4. This improves the performance for object detection in the close range, since an ultrasonic reception signal is not superimposed by a continuing oscillation of the resonant circuit formed.

Figure 2 shows a signal-time diagram to illustrate the operation of the control circuit 2 for one of the switched connections A '. It can be seen that after the semiconductor switch 24 'in question is switched off (controlled by a low level of the control signal S'), the voltage at the switched connection A' immediately rises to twice the operating voltage 2 * U _B and then due to the energy drain in the protective circuit 25 continues to rise to the limiting voltage U _G as the current flow decreases and is held there until the energy stored in the transformer is reduced. As a result, the voltage at the switched connection A' collapses down to U _B. The entire circuit should be designed so that the voltage drop at the switched connection A' occurs by the time the next pulse of the control signal S' occurs.

Reference symbol list

1

Ultrasonic transducer system

2

Control circuit

3

Receiving circuit

4

Ultrasonic transducer

5

Control unit

21

transformer

22, 22', 22"

primary side windings

23

secondary side winding

24', 24"

Semiconductor switch

25

Protection circuit

UB

Operating voltage, first supply potential

R1

Leakage resistance

R2

Resistance

C1

Storage capacity

C2

Buffer capacity

C4

capacitor

GND

second supply potential

A', A"

switched connections

D1, D2

diodes

ZD1

Zener diode

Claims (10)

Control circuit (2) for a piezoelectric ultrasonic transducer (4) in an ultrasonic transducer system (1), comprising: - a transformer (21) with at least one primary winding (22, 22', 22"); - a switching unit with a semiconductor switch (24', 24"), which is connected to the at least one primary-side winding (22, 22', 22") via a switched connection (A', A"), - a control unit (5) which is designed to alternately apply an operating voltage (U _B ) to or disconnect it from the at least one primary-side winding (22, 22', 22"), - a protective circuit (25) which is electrically coupled to the switched connection (A', A") and has a Zener diode (ZD1) which sets a switch-off voltage at the switched connection (A', A") to a limiting voltage (U _G ) which corresponds to at least twice the operating voltage. Control circuit according to claim 1, wherein the limiting voltage (U _G ) is selected from a range between 100 and 150% of the amount of twice the operating voltage (U _B ), in particular between 105 and 130% of the amount of twice the operating voltage. Control circuit according to claim 1 or 2, wherein the limiting voltage (U _G ) is determined by selecting a threshold voltage or breakdown voltage (U _ZD ) of the Zener diode (ZD1). Control circuit (2) according to claim 3, wherein the protective circuit (25) is coupled to the at least one switched connection via a diode (D1, D2), in particular directly via the diode (D1, D2), which occurs when a voltage difference occurs between the at least one switched connection (A', A") and the Zener diode (ZD1) causes a charge flow from or to the at least one switched connection (A', A") to limit a voltage change at the at least one switched connection (A', A"). Control circuit according to one of claims 1 to 4, wherein the protective circuit (25) is electrically non-capacitively coupled to the at least one switched connection (A', A"). Control circuit according to one of claims 1 to 5, wherein the protection circuit (25) has a storage capacity (C1) electrically parallel to the Zener diode (ZD1). Control circuit according to one of claims 1 to 6, wherein the operating voltage is buffered by a buffer capacitance (C2), the protective circuit (25) being coupled to the buffer capacitance (C2) via a leakage resistor (R1). Control circuit according to one of claims 1 to 7, wherein the transformer (21) has two primary-side windings (22, 22 ', 22"), which are alternately connected to and separated from the operating voltage (U _B ), so that the operating voltage ( U _B ) is always only applied to one of the primary windings (22, 22', 22"). Control circuit according to claim 8, wherein the corresponding switched connections (A', A") of the primary-side windings (22, 22', 22") are connected to the protective circuit (25), in particular via a respective diode (D1, D2). Ultrasonic transducer system (1) comprising: - a piezoelectric ultrasonic transducer (4); - A control circuit (2) according to one of claims 1 to 9, wherein a secondary winding (23) of the transformer (21) is connected to the ultrasonic transducer (4).

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